(1) Field of the Invention
The present invention relates to the field of power demand load leveling, particularly in the field of pulse width modulation of LED's for controlling the intensity of the LED's without overloading the power supply.
(2) Prior Art
On the front of many supercomputers, panels are provided with an array of LED's for indicating the processes which the computer is performing at any one time. To light the LED's in the array, control lines from different processing sections of the computer are carried via a system bus to the LED panel and are connected to the LED's through transistor switches used for enabling the LED's. When a specific control line in the system bus is sent high, the respective transistor switch is enabled so as to turn on the and activate the LED associated with that control line.
To control the intensity of the LED's within the array, pulse width modulation of the control signals transmitted by the control lines has been used to produce a series of rapid activations of the enabled LED's. This is performed by providing a modulating signal having a series of pulses as input to the gating control logic. The gating control logic enables their respective LED's when the gating control logic simultaneously receive a high input from the control lines and a high input from the modulating signal. The input from the modulating signal consists of a series of digital pulses having high and low states, with at least one change of state occurring within each period of the modulating signal. The width of a given pulse in the period of the modulating signal determines the length of time that the enabled LED's will be activated during the period. The period is in the order of microseconds so that the series of activations of the LED's produce an appearance that the LED's are being dimmed rather than being turned on and off.
The gating control logic for each LED in the array receives the same modulating signal so that each enabled LED is turned on and off at the same time and at the same rate as the other enabled LED's as determined by the pulse within the period of the modulating signal. The rapid succession of activations produced by the modulating signal thereby causes each of the enabled LED's to have the same intensity output. For a small number of LED's in the array, this type of pulse width modulation switching presents no problem for the power source since the current demand on the power supply is usually in the order of several amps.
However, for larger arrays of LED's, the total amount of current required to activate all of the LED's can be in the order of 50 to 100 amps. Because the conventional method of pulse width modulation switching of LED's activates all enabled LED's at once upon the rising edge of a pulse in the modulating signal, an enormous current demand is produced when a significant percentage of the LED's is enabled. Each time this occurs, the sudden change in current demand may overload the power supply and trigger the power supply over current detection which shuts down the power supply. In addition, the power supply may have severe difficulty in maintaining a regulated voltage during these high current switching transitions. When the power supply responds to a sudden change in the current demand the output voltage will change in the inverse direction of the current demand, especially with switching power supplies.
Accordingly, it is an object of the present invention to provide a method and apparatus for pulse width modulation of LED's that reduces the magnitude of the instantaneous change in the current demand on the power supply such that a more stable output voltage can be maintained.
It is another object of the present invention to provide a method and apparatus for pulse width modulation of an array of LED's in which only twenty five percent of the maximum current demand is switched at any one time.